1. Field of the Invention
The present invention relates to an electrical circuit for wave shaping and, more particularly, it pertains to a hysteresis comparator having enhanced low frequency noise immunity.
2. Description of the Prior Art
Schmidt trigger circuits, or, equivalently, voltage-controlled bistable multivibrators, are known for their applicability as wave shaping circuits. A discussion of these circuits is given in Giacoletto, L. J., Electronic Designers' Handbook, 2d Ed., New York, McGraw-Hill Book Co., 1977, Sec. 18.72, Sec. 19, and Sec. 20.6. With sufficient regenerative feedback, the transfer function of these circuits is a hysteresis loop. In this case, the Schmidt trigger circuit is termed a hysteresis comparator. The hysteresis loop characteristic of such a comparator gives the circuit a high degree of noise immunity. However, when the noise and signal components of an input signal have approximately the same voltage excursions, adjusting the width of the input comparison region of a hysteresis comparator to eliminate noise increases the risk that information will be lost as well.
Hysteresis comparators have found application in command receivers which provide remote controlled actuation of mechanisms upon the receipt of a predetermined coded signal. These receivers are used in such applications as, for example, automatic garage door openers. An early version of such a command receiver is disclosed in U.S. Pat. No. 3,359,558, issued Dec. 19, 1967 to the inventor of the present invention.
Present day garage door opening systems typically use pulse-width-modulated coded signals at 250 bits per second for the actuation command. In a preferred embodiment of a command receiver the coded signal is detected, wave-shaped, and then decoded in digital decoding circuitry. The wave shaper serves to decrease the probability that the decoding circuitry will respond to noise and to increase the probability the decoder will respond to a valid signal. A hysteresis comparator used as the signal wave shaper for driving the decoder will block the transmission of all voltage excursions, and thus all noise, having a peak-to-peak amplitude less than the width of the hysteresis loop.
High sensitivity is desirable in a command receiver for increasing the range at which the remotely controlled actuator can respond to a transmitter. Stated alternatively, command receivers are typically designed to have high sensitivity so that the transmitters used in the system need have only relatively low-level radiation output. The high sensitivity is typically achieved through the use of a self-quenching superregenerative detector as the first stage in the receiver. This type of detector has a relatively very high gain.
A problem which arises in typical relatively low-cost production models of command receivers is due to the amplification in the detector of the 60 hertz (power frequency) hum produced by the power supply in the receiver.
The hum input to the wave shaper can have an amplitude which is approximately equal to the amplitude at the detected pulse-width modulated signal. Thus, a design for a hysteresis comparator having an input comparison region (i.e., the region defined by the difference between the two switching levels of the hysteresis loop) sufficiently wide to exclude the 60 hertz hum may also tend to exclude a coded signal to which a response is desired.
Thus, there is a need to include means in a command receiver to reduce the effect of power supply hum. With the effect of power supply hum reduced, switching levels of a hysteresis comparator in the receiver can be set for more reliable error-free response to transmitted signals at greater transmitter ranges.